Polyacrylamide has poor salt-resistance, and it must be combined with freshwater so as to reach economic and stable viscosity for the polymer solution. Therefore, it turns into one of the hot spots of study in the world's petroleum chemical field.
Polymer flooding, which has been used as an important part of the technologies for increasing oil fields' petroleum yield since 1950s, is to introduce polymer into water so as to increase the water viscosity and lower the water fluidity. Increased viscosity and decreased aqueous phase permeability caused by the use of certain polymers leads to decrease of the fluidity ratio, which increases the volume spread efficiency and decreases the oil saturation in the spread zone so as to promote the water-flooding efficiency.
It is well known that the polymers used for polymer-flooding oil are mainly partially hydrolyzed polyacrylamide and polysaccharide (biopolymer). However, in freshwater, the molecules of polyacrylamide are in a stretched sate with a strong tackifying ability because of electrical mutual exclusion among the sodium carboxyl groups in the molecules; while in brine, the molecules of polyacrylamide are in a curly state because the sodium carboxyl groups in the polyacrylamide molecules are electrically shielded. The higher the degree of hydrolysis (i.e. the higher the content of sodium carboxyl groups), the polyacrylamide molecules in brine are curlier, and the tackifying ability is poorer. When hydrolysis degree of polyacrylamide is equal to or greater than 40%, deposition will not occur, in spite of seriously curved polyacrylamide molecules and greatly decreased tackifying ability. In hard water (with high contents of Ca2+ and Mg2+), when hydrolysis degree of polyacrylamide is equal to or greater than 40%, polyacrylamide molecules combine with polyvalent ions such as calcium and magnesium, thus flocculating deposition will occur. Polymer stability is very important because of long period of tertiary oil extraction cycles. Therefore, the polymer for tertiary oil extraction must ensure the hydrolysis degree of molecules thereof is ≦40% in over three months in oil field stratum, only so, the polymer can possess heat-resistant and salt-resistant properties in oil field applications. However, hydrolysis reaction of the amide radical of polyacrylamide is very fast in acid or alkali conditions, and it will also be accelerated quickly in neutral conditions by rising of temperature, which causes the polyacrylamide to lose its heat-resistant and salt-resistant properties in oil reservoir temperature, and it might also causes harms to the stratum due to jamming the stratum. Polysaccharide has good heat-resistance and salt-resistance, but it has the shortcomings of high cost, poor feeding properties, and being easy to be biodegraded.
A lot of research work has been done in order to overcome the aforementioned shortcomings. U.S. Pat. No. 4,304,902 discloses an ethylene oxide polymer with long-chain epoxide, but this method needs high concentration of the said polymer (about 1%) for thickening, and it also demands surfactant to assist dissolution. U.S. Pat. No. 4,814,096 discloses that the copolymer of acrylamide, acrylic acid and dodecyl methacrylate (i.e., a hydrophobic associated polymer) have good heat-resistance, salt-resistance and anti-shearing properties. The disadvantage of this method lies in that the dodecyl methacrylate is water-insoluble. A large quantity of surfactants must be introduced in for copolymerization, which on the one hand leads to high cost for the copolymer, and on the other hand is difficult to provide a polymer of high molecular weight, so that its capability for thickening aqueous medium is poor and the applications are limited with dramatically increased application cost.
With extensive literature study, we classify the researches on heat-resistant and salt-resistant polymers both at home and abroad into amphoteric polymers, monomeric heat-resistant and salt-resistant polymers, hydrophobic associated polymers, complex polymers, blending polymers and comb-shaped polymers. But the amphoteric polymers have poor solubility due to internally complex structure of anion and cation radicals in the molecules. As the aging time increases, the hydrolysis degree of amphoteric polymers containing acrylamide increases, the positive-charge and negative-charge radicals on the molecule chains become unequal, the molecule chains become curlier and curlier as the mineralization develops, and the solution viscosity highly drops and salt-resistance disappears gradually. Besides, cations of the amphoteric polymers can lead to highly increased adsorption amount of polymers in the stratum, and the polymers are massively adsorbed in regions near the oil wells so as to seriously affect the tertiary oil extraction efficiency adversely and increase the tertiary oil extraction cost. Monomeric heat-resistant and salt-resistant polymers truly have long term heat-resistance and salt-resistance, but it costs too much to manufacture the polymers with the currently available production conditions (synthesis of raw materials, synthesis method and production techniques), so they can be used only in some special cases, and no one can afford it economically to apply the polymers in large scale of tertiary oil extraction. Hydrophobic associated polymers produce intra- and inter-molecular association, and according to analysis to probability and molecule's formation stability, the intra-molecular association should be superior to the inter-molecular association. Therefore, the hydrophobic associated polymer solution is very poor in stability. The viscosity of the polymer solution will drop rapidly with extension of test time, and there even exist the separation of association (i.e., deposition). And similarly, the hydrophobic associated polymers are poor in water solubility, but with high filtration factor. They appear in the form of glue products and it is hard to turn them in a economic way into a water-soluble powder product to meet the requirement of oil well's tertiary oil extraction (it can promote the polymers' solubility to add large quantity of surfactant before drying, but the hydrophobic association properties of the polymers will be seriously affected adversely and the production cost will increase). Rising of temperature can promote hydrophobicity of the hydrophobic radicals and enhance the hydrophobic association properties, but at the same time it accelerates the speed of intra-molecular association, thus the stability of the solution becomes poor. In addition, the sequential distribution of hydrophobic monomeric units along the main polymer molecular chains is also an important parameter with respect to the solution's viscosity. The products of non-homogeneous polymerization and homogeneous polymerization are of random structures, that is to say, it is hard to control the product quality of the hydrophobic associated polymers (micelle polymerization process can help settle the issue, but it will increase the production cost abundantly). The short wire-drawing behavior (poor in flexibility) of the hydrophobic associated polymers degrades the elastic oil flooding effects (which are equivalent to viscosity oil flooding effects), therefore, the polymers' ability of oil flooding is affected adversely. Hydrophobic associated polymers have relatively low molecular weights, so association only occurs in high concentration rather than in low concentration, and in such an occasion, the solution viscosity is much lower than that of conventional polyacrylamide. That is to say, hydrophobic associated polymers cannot resist the dilution of stratum water. However, large quantity of water exists in tertiary oil extraction stratum. Electrolyte with small molecules can enhance the solvent's polarity so as to enhance the hydrophobic association. This will leads to intensifying the polymer's intra-molecular association in condition of high degree of mineralization. That is to say, variation of mineralization degree can influence the hydrophobic associated polymers greatly, and hydrophobic associated polymers cannot resist high salt content. The complex polymers based on combinations of aforementioned principles have better properties than single of the foregoing polymers, and the application scopes thereof are expanded further. However, they still cannot overcome the defects of aforementioned polymers on principle, so they cannot meet the requirements for tertiary oil extraction in oil field.
A Chinese patent application with invention No. 01136798.9 has been disclosed for a comb-shaped salt-resistant thickening agent (a comb-shaped polymer) in the past, and the thickening agent has been extensively used in oil reservoir of class I. It is formulated with sewage directly and its thickening ability is 50% greater than that of polyacrylamide. The oil condensing and water decreasing effects thereof are very remarkable, and it has become a new oil flooding agent for tertiary oil extraction for class I oil reservoir (with good penetrability). However, the molecular weight of the industrial products of these comb-shaped polymers is greater than that of polyacrylamide, and its dissolving rate is slower than that of polyacrylamide, so it will encounter some troubles in injection properties when it is used for class II oil reservoirs (of which the penetrability is poorer than that of class I oil reservoir). Tertiary oil extraction from class II oil reservoirs is becoming an emphasis for oil fields. The penetrability of class II oil reservoirs is lower than that of class I oil reservoir, so it is necessary to utilize a new salt-resistant polymer that has lower molecular weight, good salt-resistance, and a dissolving rate the same as that of polyacrylamide.